<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="generator" content="Asciidoctor 2.0.26">
<link rel="icon" type="image/svg" href="../../../../_docs/_Assets/Logo.svg">
<title>Data Validation</title>
<style>
@import url(https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.5.1/css/all.min.css);

:root {
    --background: #fff;
    --black-color: #000;
    --background-yellow: #ff0;
    --border: #c0c0c0;
    --gray-color: #6c818f;
    --link-color-default: var(--accent-primary-color);
    --foreground-color: #222222;
    --accent-primary-color: #3578e5;
    --accent-secondary-color: #102445;
    --border2: #dddddd;
    --block-quote-color: #748590;
    --pre-background-color: #f2f2f2;
    --dark-gray-color: #333333;
    --kdb-background-color: #F7F7F7;
    --header-color: var(--accent-primary-color);
    --link-hover-color: #102445;
    --toc-border-color: #d9d9d9;
    --exampleblock-border-color: #e6e6e6;
    --em-color: #373737;
    --admotion-background-color: #e6e6e6;
    --sidebar-background-color: #e8ecef;
    --border-radius-default: 6px;
}

/* normalize.css v2.1.1 | MIT License | git.io/normalize */
/* ========================================================================== HTML5 display definitions ========================================================================== */
/** Correct `block` display not defined in IE 8/9. */
article, aside, details, figcaption, figure, footer, header, hgroup, main, nav, section, summary {
    display: block;
}

/** Correct `inline-block` display not defined in IE 8/9. */
audio, canvas, video {
    display: inline-block;
}

/** Prevent modern browsers from displaying `audio` without controls. Remove excess height in iOS 5 devices. */
audio:not([controls]) {
    display: none;
    height: 0;
}

/** Address styling not present in IE 8/9. */
[hidden] {
    display: none;
}

/* ========================================================================== Base ========================================================================== */
/** 1. Prevent system color scheme's background color being used in Firefox, IE, and Opera. 2. Prevent system color scheme's text color being used in Firefox, IE, and Opera. 3. Set default font family to sans-serif. 4. Prevent iOS text size adjust after orientation change, without disabling user zoom. */
html {
    background: var(--background); /* 1 */
    color: var(--black-color); /* 2 */
    font-family: sans-serif; /* 3 */
    -ms-text-size-adjust: 100%; /* 4 */
    -webkit-text-size-adjust: 100%; /* 4 */
}

/** Remove default margin. */
body {
    margin: 0;
}

/* ========================================================================== Links ========================================================================== */
/** Address `outline` inconsistency between Chrome and other browsers. */
a:focus {
    outline: thin dotted;
}

/** Improve readability when focused and also mouse hovered in all browsers. */
a:active, a:hover {
    outline: 0;
}

/* ========================================================================== Typography ========================================================================== */
/** Address variable `h1` font-size and margin within `section` and `article` contexts in Firefox 4+, Safari 5, and Chrome. */
h1 {
    font-size: 2em;
    margin: 0.67em 0;
}

/** Address styling not present in IE 8/9, Safari 5, and Chrome. */
abbr[title] {
    border-bottom: 1px dotted;
}

/** Address style set to `bolder` in Firefox 4+, Safari 5, and Chrome. */
b, strong {
    font-weight: bold;
}

/** Address styling not present in Safari 5 and Chrome. */
dfn {
    font-style: italic;
}

/** Address differences between Firefox and other browsers. */
hr {
    -moz-box-sizing: content-box;
    box-sizing: content-box;
    height: 0;
}

/** Address styling not present in IE 8/9. */
mark {
    background: var(--background-yellow);
    color: var(--black-color);
}

/** Correct font family set oddly in Safari 5 and Chrome. */
code, kbd, pre, samp {
    font-family: monospace, serif;
    font-size: 1em;
}

/** Improve readability of pre-formatted text in all browsers. */
pre {
    white-space: pre-wrap;
}

/** Set consistent quote types. */
q {
    quotes: "\201C" "\201D" "\2018" "\2019";
}

/** Address inconsistent and variable font size in all browsers. */
small {
    font-size: 80%;
}

/** Prevent `sub` and `sup` affecting `line-height` in all browsers. */
sub, sup {
    font-size: 75%;
    line-height: 0;
    position: relative;
    vertical-align: baseline;
}

sup {
    top: -0.5em;
}

sub {
    bottom: -0.25em;
}

/* ========================================================================== Embedded content ========================================================================== */
/** Remove border when inside `a` element in IE 8/9. */
img {
    border: 0;
}

/** Correct overflow displayed oddly in IE 9. */
svg:not(:root) {
    overflow: hidden;
}

/* ========================================================================== Figures ========================================================================== */
/** Address margin not present in IE 8/9 and Safari 5. */
figure {
    margin: 0;
}

/* ========================================================================== Forms ========================================================================== */
/** Define consistent border, margin, and padding. */
fieldset {
    border: 1px solid var(--border);
    margin: 0 2px;
    padding: 0.35em 0.625em 0.75em;
}

/** 1. Correct `color` not being inherited in IE 8/9. 2. Remove padding so people aren't caught out if they zero out fieldsets. */
legend {
    border: 0; /* 1 */
    padding: 0; /* 2 */
}

/** 1. Correct font family not being inherited in all browsers. 2. Correct font size not being inherited in all browsers. 3. Address margins set differently in Firefox 4+, Safari 5, and Chrome. */
button, input, select, textarea {
    font-family: inherit; /* 1 */
    font-size: 100%; /* 2 */
    margin: 0; /* 3 */
}

/** Address Firefox 4+ setting `line-height` on `input` using `!important` in the UA stylesheet. */
button, input {
    line-height: normal;
}

/** Address inconsistent `text-transform` inheritance for `button` and `select`. All other form control elements do not inherit `text-transform` values. Correct `button` style inheritance in Chrome, Safari 5+, and IE 8+. Correct `select` style inheritance in Firefox 4+ and Opera. */
button, select {
    text-transform: none;
}

/** 1. Avoid the WebKit bug in Android 4.0.* where (2) destroys native `audio` and `video` controls. 2. Correct inability to style clickable `input` types in iOS. 3. Improve usability and consistency of cursor style between image-type `input` and others. */
button, html input[type="button"], input[type="reset"], input[type="submit"] {
    -webkit-appearance: button; /* 2 */
    cursor: pointer; /* 3 */
}

/** Re-set default cursor for disabled elements. */
button[disabled], html input[disabled] {
    cursor: default;
}

/** 1. Address box sizing set to `content-box` in IE 8/9. 2. Remove excess padding in IE 8/9. */
input[type="checkbox"], input[type="radio"] {
    box-sizing: border-box; /* 1 */
    padding: 0; /* 2 */
}

/** 1. Address `appearance` set to `searchfield` in Safari 5 and Chrome. 2. Address `box-sizing` set to `border-box` in Safari 5 and Chrome (include `-moz` to future-proof). */
input[type="search"] {
    -webkit-appearance: textfield; /* 1 */
    -moz-box-sizing: content-box;
    -webkit-box-sizing: content-box; /* 2 */
    box-sizing: content-box;
}

/** Remove inner padding and search cancel button in Safari 5 and Chrome on OS X. */
input[type="search"]::-webkit-search-cancel-button, input[type="search"]::-webkit-search-decoration {
    -webkit-appearance: none;
}

/** Remove inner padding and border in Firefox 4+. */
button::-moz-focus-inner, input::-moz-focus-inner {
    border: 0;
    padding: 0;
}

/** 1. Remove default vertical scrollbar in IE 8/9. 2. Improve readability and alignment in all browsers. */
textarea {
    overflow: auto; /* 1 */
    vertical-align: top; /* 2 */
}

/* ========================================================================== Tables ========================================================================== */
/** Remove most spacing between table cells. */
table {
    border-collapse: collapse;
    border-spacing: 0;
}

*, *:before, *:after {
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
    box-sizing: border-box;
}

html, body {
    font-size: 100%;
}

body {
    background: white;
    color: var(--foreground-color);
    padding: 0;
    margin: 0;
    font-family: "Helvetica Neue", "Helvetica", Helvetica, Arial, sans-serif;
    font-weight: normal;
    font-style: normal;
    line-height: 1;
    position: relative;
    cursor: auto;
}

a:hover {
    cursor: pointer;
}

a:focus {
    outline: none;
}

img, object, embed {
    max-width: 100%;
    height: auto;
}

object, embed {
    height: 100%;
}

img {
    -ms-interpolation-mode: bicubic;
}

#map_canvas img, #map_canvas embed, #map_canvas object, .map_canvas img, .map_canvas embed, .map_canvas object {
    max-width: none !important;
}

.left {
    float: left !important;
}

.right {
    float: right !important;
}

.text-left {
    text-align: left !important;
}

.text-right {
    text-align: right !important;
}

.text-center {
    text-align: center !important;
}

.text-justify {
    text-align: justify !important;
}

.hide {
    display: none;
}

.antialiased, body {
    -webkit-font-smoothing: antialiased;
}

img {
    display: inline-block;
    vertical-align: middle;
}

textarea {
    height: auto;
    min-height: 50px;
}

select {
    width: 100%;
}

p.lead, .paragraph.lead > p, #preamble > .sectionbody > .paragraph:first-of-type p {
    font-size: 1.21875em;
    line-height: 1.6;
}

.subheader, #content #toctitle, .admonitionblock td.content > .title, .exampleblock > .title, .imageblock > .title, .videoblock > .title, .listingblock > .title, .literalblock > .title, .openblock > .title, .paragraph > .title, .quoteblock > .title, .sidebarblock > .title, .tableblock > .title, .verseblock > .title, .dlist > .title, .olist > .title, .ulist > .title, .qlist > .title, .hdlist > .title, .tableblock > caption {
    line-height: 1.4;
    color: var(--accent-primary-color);
    font-weight: 300;
    margin-top: 0.5em;
    margin-bottom: 0.5em;
}

/* Typography resets */
div, dl, dt, dd, ul, ol, li, h1, h2, h3, #toctitle, .sidebarblock > .content > .title, h4, h5, h6, pre, form, p, blockquote, th, td {
    margin: 0;
    /*padding: 0;*/
    direction: ltr;
}

/* Default Link Styles */
a {
    color: var(--link-color-default);
    text-decoration: none;
    line-height: inherit;
}

a:hover, a:focus {
    color: var(--header-color);
}

a img {
    border: none;
}

/* Default paragraph styles */
p {
    font-family: inherit;
    font-weight: normal;
    font-size: 1em;
    line-height: 1.5;
    margin-bottom: 1.25em;
    text-rendering: optimizeLegibility;
}

p aside {
    font-size: 0.875em;
    line-height: 1.35;
    font-style: italic;
}

/* Default header styles */
h1, h2, h3, #toctitle, .sidebarblock > .content > .title, h4, h5, h6 {
    font-family: ff-meta-web-pro-1, ff-meta-web-pro-2, Arial, "Helvetica Neue", sans-serif;
    font-weight: bold;
    font-style: normal;
    color: var(--accent-primary-color);
    text-rendering: optimizeLegibility;
    margin-top: 1em;
    margin-bottom: 0.5em;
    line-height: 1.2125em;
}

h1 small, h2 small, h3 small, #toctitle small, .sidebarblock > .content > .title small, h4 small, h5 small, h6 small {
    font-size: 60%;
    color: var(--accent-secondary-color);
    line-height: 0;
}

h1 {
    font-size: 2.125em;
}

h2 {
    font-size: 1.6875em;
}

h3, #toctitle, .sidebarblock > .content > .title {
    font-size: 1.375em;
}

h4 {
    font-size: 1.125em;
}

h5 {
    font-size: 1.125em;
}

h6 {
    font-size: 1em;
}

hr {
    border: solid var(--border2);
    border-width: 1px 0 0;
    clear: both;
    margin: 1.25em 0 1.1875em;
    height: 0;
}

/* Helpful Typography Defaults */
em, i {
    font-style: italic;
    line-height: inherit;
}

strong, b {
    font-weight: bold;
    line-height: inherit;
}

small {
    font-size: 60%;
    line-height: inherit;
}

code {
    font-family: "Consolas", "Deja Vu Sans Mono", "Bitstream Vera Sans Mono", monospace;
    font-weight: normal;
    color: var(--link-color-default);
}

/* Lists */
ul, ol, dl {
    font-size: 1em;
    line-height: 1.5;
    margin-bottom: 1.25em;
    list-style-position: outside;
    font-family: inherit;
}

ul, ol {
    margin-left: 0;
}

/* Unordered Lists */
ul li ul, ul li ol {
    margin-left: 1.25em;
    margin-bottom: 0;
    font-size: 1em; /* Override nested font-size change */
}

ul.square li ul, ul.circle li ul, ul.disc li ul {
    list-style: inherit;
}

ul.square {
    list-style-type: square;
}

ul.circle {
    list-style-type: circle;
}

ul.disc {
    list-style-type: disc;
}

ul.no-bullet {
    list-style: none;
}

/* Ordered Lists */
ol li ul, ol li ol {
    margin-left: 1.25em;
    margin-bottom: 0;
}

/* Definition Lists */
dl dt {
    margin-bottom: 0.3em;
    font-weight: bold;
}

dl dd {
    margin-bottom: 0.75em;
}

/* Abbreviations */
abbr, acronym {
    text-transform: uppercase;
    font-size: 90%;
    color: black;
    border-bottom: 1px dotted var(--border2);
    cursor: help;
}

abbr {
    text-transform: none;
}

/* Blockquotes */
blockquote {
    margin: 0 0 1.25em;
    padding: 0.5625em 1.25em 0 1.1875em;
    border-left: 1px solid var(--border2);
}

blockquote cite {
    display: block;
    font-size: 0.8125em;
    color: var(--block-quote-color);
}

blockquote cite:before {
    content: "\2014 \0020";
}

blockquote cite a, blockquote cite a:visited {
    color: var(--block-quote-color);
}

blockquote, blockquote p {
    line-height: 1.5;
    color: var(--accent-secondary-color);
}

/* Microformats */
.vcard {
    display: inline-block;
    margin: 0 0 1.25em 0;
    border: 1px solid var(--border2);
    padding: 0.625em 0.75em;
}

.vcard li {
    margin: 0;
    display: block;
}

.vcard .fn {
    font-weight: bold;
    font-size: 0.9375em;
}

.vevent .summary {
    font-weight: bold;
}

.vevent abbr {
    cursor: auto;
    text-decoration: none;
    font-weight: bold;
    border: none;
    padding: 0 0.0625em;
}

@media only screen and (min-width: 768px) {
    h1, h2, h3, #toctitle, .sidebarblock > .content > .title, h4, h5, h6 {
        line-height: 1.4;
    }

    h1 {
        font-size: 2.75em;
    }

    h2 {
        font-size: 2.3125em;
    }

    h3, #toctitle, .sidebarblock > .content > .title {
        font-size: 1.6875em;
    }

    h4 {
        font-size: 1.4375em;
    }
}

/* Print styles.  Inlined to avoid required HTTP connection: www.phpied.com/delay-loading-your-print-css/ Credit to Paul Irish and HTML5 Boilerplate (html5boilerplate.com)
*/
.print-only {
    display: none !important;
}

@media print {
    * {
        background: transparent !important;
        color: var(--black-color) !important; /* Black prints faster: h5bp.com/s */
        box-shadow: none !important;
        text-shadow: none !important;
    }

    a, a:visited {
        text-decoration: underline;
    }

    a[href]:after {
        content: " (" attr(href) ")";
    }

    abbr[title]:after {
        content: " (" attr(title) ")";
    }

    .ir a:after, a[href^="javascript:"]:after, a[href^="#"]:after {
        content: "";
    }

    pre, blockquote {
        border: 1px solid #999;
        page-break-inside: avoid;
    }

    thead {
        display: table-header-group; /* h5bp.com/t */
    }

    tr, img {
        page-break-inside: avoid;
    }

    img {
        max-width: 100% !important;
    }

    @page {
        margin: 0.5cm;
    }

    p, h2, h3, #toctitle, .sidebarblock > .content > .title {
        orphans: 3;
        widows: 3;
    }

    h2, h3, #toctitle, .sidebarblock > .content > .title {
        page-break-after: avoid;
    }

    .hide-on-print {
        display: none !important;
    }

    .print-only {
        display: block !important;
    }

    .hide-for-print {
        display: none !important;
    }

    .show-for-print {
        display: inherit !important;
    }
}

/* Tables */
table {
    background: white;
    margin-bottom: 1.25em;
    border: solid 0 var(--border2);
}

table thead, table tfoot {
    background: none;
    font-weight: bold;
}

table thead tr th, table thead tr td, table tfoot tr th, table tfoot tr td {
    padding: 1px 8px 1px 5px;
    font-size: 1em;
    color: var(--foreground-color);
    text-align: left;
}

table tr th, table tr td {
    padding: 1px 8px 1px 5px;
    font-size: 1em;
    color: var(--foreground-color);
}

table tr.even, table tr.alt, table tr:nth-of-type(even) {
    background: none;
}

table thead tr th, table tfoot tr th, table tbody tr td, table tr td, table tfoot tr td {
    display: table-cell;
    line-height: 1.5;
}

.clearfix:before, .clearfix:after, .float-group:before, .float-group:after {
    content: " ";
    display: table;
}

.clearfix:after, .float-group:after {
    clear: both;
}

*:not(pre) > code {
    font-size: 0.95em;
    padding: 0;
    white-space: nowrap;
    background-color: var(--pre-background-color);
    border: 0 solid var(--border2);
    -webkit-border-radius: var(--border-radius-default);
    border-radius: var(--border-radius-default);
    text-shadow: none;
}

pre, pre > code {
    line-height: 1.2;
    color: inherit;
    font-family: "Consolas", "Deja Vu Sans Mono", "Bitstream Vera Sans Mono", monospace;
    font-weight: normal;
}

kbd.keyseq {
    color: var(--dark-gray-color);
}

kbd:not(.keyseq) {
    display: inline-block;
    color: black;
    font-size: 0.75em;
    line-height: 1.4;
    background-color: var(--kdb-background-color);
    border: 1px solid #ccc;
    -webkit-border-radius: 3px;
    border-radius: 3px;
    -webkit-box-shadow: 0 1px 0 rgba(0, 0, 0, 0.2), 0 0 0 2px white inset;
    box-shadow: 0 1px 0 rgba(0, 0, 0, 0.2), 0 0 0 2px white inset;
    margin: -0.15em 0.15em 0 0.15em;
    padding: 0.2em 0.6em 0.2em 0.5em;
    vertical-align: middle;
    white-space: nowrap;
}

kbd kbd:first-child {
    margin-left: 0;
}

kbd kbd:last-child {
    margin-right: 0;
}

.menuseq, .menu {
    color: black;
}

p a > code:hover {
    color: var(--link-hover-color);
}

#header, #content, #footnotes, #footer {
    width: 100%;
    margin-left: auto;
    margin-right: auto;
    margin-top: 0;
    margin-bottom: 0;
    max-width: 62.5em;
    *zoom: 1;
    position: relative;
    padding-left: 0.9375em;
    padding-right: 0.9375em;
}

#header:before, #header:after, #content:before, #content:after, #footnotes:before, #footnotes:after, #footer:before, #footer:after {
    content: " ";
    display: table;
}

#header:after, #content:after, #footnotes:after, #footer:after {
    clear: both;
}

#header {
    margin-bottom: 2.5em;
}

#header > h1 {
    color: var(--header-color);
    font-weight: bold;
    border-bottom: 1px solid var(--border2);
    margin-bottom: -28px;
    padding-bottom: 32px;
}

#header span {
    color: var(--accent-secondary-color);
}

#header #revnumber {
    text-transform: capitalize;
}

#header br {
    display: none;
}

#header br + span {
    padding-left: 3px;
}

#header br + span:before {
    content: "\2013 \0020";
}

#header br + span.author {
    padding-left: 0;
}

#header br + span.author:before {
    content: ", ";
}

#toc {
    border-bottom: 1px solid var(--border2);
    padding-bottom: 1.25em;
}

#toc > ul {
    margin-left: 0.25em;
}

#toc ul.sectlevel0 > li > a {
    font-style: italic;
}

#toc ul.sectlevel0 ul.sectlevel1 {
    margin-left: 0;
    margin-top: 0.5em;
    margin-bottom: 0.5em;
}

#toc ul {
    list-style-type: none;
}

#toctitle {
    color: var(--accent-primary-color);
}

@media only screen and (min-width: 1280px) {
    body.toc2 {
        padding-left: 20em;
    }

    #toc.toc2 {
        position: fixed;
        width: 20em;
        left: 0;
        top: 0;
        border-right: 1px solid var(--border2);
        border-bottom: 0;
        z-index: 1000;
        padding: 44px 1em 1em;
        height: 100%;
        overflow: auto;
    }

    #toc.toc2 > ul {
        padding-left: 1em;
    }

    #toc.toc2 #toctitle {
        margin-top: 0;
    }

    #toc.toc2 > ul {
        font-size: .95em;
    }

    #toc.toc2 ul ul {
        margin-left: 0;
        padding-left: 1.25em;
    }

    #toc.toc2 ul.sectlevel0 ul.sectlevel1 {
        padding-left: 0;
        margin-top: 0.5em;
        margin-bottom: 0.5em;
    }

    body.toc2.toc-right {
        padding-left: 0;
        padding-right: 20em;
    }

    body.toc2.toc-right #toc.toc2 {
        border-right: 0;
        border-left: 1px solid var(--border2);
        left: auto;
        right: 0;
    }
}

#content #toc {
    border-style: solid;
    border-width: 1px;
    border-color: var(--toc-border-color);
    margin-bottom: 1.25em;
    padding: 1.25em;
    background: var(--pre-background-color);
    border-width: 0;
    -webkit-border-radius: var(--border-radius-default);
    border-radius: var(--border-radius-default);
}

#content #toc > :first-child {
    margin-top: 0;
}

#content #toc > :last-child {
    margin-bottom: 0;
}

#content #toc a {
    color: var(--gray-color);
    text-decoration: none;
}

#content #toctitle {
    font-weight: bold;
    font-family: ff-meta-web-pro-1, ff-meta-web-pro-2, Arial, "Helvetica Neue", sans-serif;
    font-size: 1em;
    padding-left: 0.125em;
}

#footer {
    max-width: 100%;
    background-color: black;
    padding: 1.25em;
}

#footer-text {
    color: white;
    line-height: 1.35;
}

.sect1 {
    padding-bottom: 1.25em;
}

.sect1 + .sect1 {
    border-top: 1px solid var(--border2);
}

#content h1 > a.anchor, h2 > a.anchor, h3 > a.anchor, #toctitle > a.anchor, .sidebarblock > .content > .title > a.anchor, h4 > a.anchor, h5 > a.anchor, h6 > a.anchor {
    position: absolute;
    width: 1em;
    margin-left: -1em;
    display: block;
    text-decoration: none;
    visibility: hidden;
    text-align: center;
    font-weight: normal;
}

#content h1 > a.anchor:before, h2 > a.anchor:before, h3 > a.anchor:before, #toctitle > a.anchor:before, .sidebarblock > .content > .title > a.anchor:before, h4 > a.anchor:before, h5 > a.anchor:before, h6 > a.anchor:before {
    content: '\00A7';
    font-size: .85em;
    vertical-align: text-top;
    display: block;
    margin-top: 0.05em;
}

#content h1:hover > a.anchor, #content h1 > a.anchor:hover, h2:hover > a.anchor, h2 > a.anchor:hover, h3:hover > a.anchor, #toctitle:hover > a.anchor, .sidebarblock > .content > .title:hover > a.anchor, h3 > a.anchor:hover, #toctitle > a.anchor:hover, .sidebarblock > .content > .title > a.anchor:hover, h4:hover > a.anchor, h4 > a.anchor:hover, h5:hover > a.anchor, h5 > a.anchor:hover, h6:hover > a.anchor, h6 > a.anchor:hover {
    visibility: visible;
}

#content h1 > a.link, h2 > a.link, h3 > a.link, #toctitle > a.link, .sidebarblock > .content > .title > a.link, h4 > a.link, h5 > a.link, h6 > a.link {
    color: var(--accent-primary-color);
    text-decoration: none;
}

#content h1 > a.link:hover, h2 > a.link:hover, h3 > a.link:hover, #toctitle > a.link:hover, .sidebarblock > .content > .title > a.link:hover, h4 > a.link:hover, h5 > a.link:hover, h6 > a.link:hover {
    color: var(--link-hover-color);
}

.imageblock, .literalblock, .listingblock, .verseblock, .videoblock {
    margin-bottom: 1.25em;
}

.admonitionblock td.content > .title, .exampleblock > .title, .imageblock > .title, .videoblock > .title, .listingblock > .title, .literalblock > .title, .openblock > .title, .paragraph > .title, .quoteblock > .title, .sidebarblock > .title, .tableblock > .title, .verseblock > .title, .dlist > .title, .olist > .title, .ulist > .title, .qlist > .title, .hdlist > .title {
    text-align: left;
    font-weight: bold;
}

.tableblock > caption {
    text-align: left;
    font-weight: bold;
    white-space: nowrap;
    overflow: visible;
    max-width: 0;
}

table.tableblock #preamble > .sectionbody > .paragraph:first-of-type p {
    font-size: inherit;
}

.admonitionblock > table {
    border: 0;
    background: none;
    width: 100%;
}

.admonitionblock > table td.icon {
    text-align: center;
    width: 60px;
    padding: 0;
    font-size: 1.5em;
}

.admonitionblock > table td.icon img {
    max-width: none;
}

.admonitionblock > table td.icon .title {
    font-weight: bold;
    text-transform: uppercase;
}

.admonitionblock > table td.content {
    /*padding-left: 1.125em;*/
    /*padding-right: 1.25em;*/
    border-left: 1px solid var(--border2);
    color: var(--accent-secondary-color);
}

.admonitionblock > table td.content > :last-child > :last-child {
    margin-bottom: 0;
}

.exampleblock > .content {
    border-style: solid;
    border-width: 1px;
    border-color: var(--exampleblock-border-color);
    margin-bottom: 1.25em;
    padding: 1.25em;
    background: white;
    -webkit-border-radius: var(--border-radius-default);
    border-radius: var(--border-radius-default);
}

.exampleblock > .content > :first-child {
    margin-top: 0;
}

.exampleblock > .content > :last-child {
    margin-bottom: 0;
}

.exampleblock > .content h1, .exampleblock > .content h2, .exampleblock > .content h3, .exampleblock > .content #toctitle, .sidebarblock.exampleblock > .content > .title, .exampleblock > .content h4, .exampleblock > .content h5, .exampleblock > .content h6, .exampleblock > .content p {
    color: var(--dark-gray-color);
}

.exampleblock > .content h1, .exampleblock > .content h2, .exampleblock > .content h3, .exampleblock > .content #toctitle, .sidebarblock.exampleblock > .content > .title, .exampleblock > .content h4, .exampleblock > .content h5, .exampleblock > .content h6 {
    line-height: 1;
    margin-bottom: 0.625em;
}

.exampleblock > .content h1.subheader, .exampleblock > .content h2.subheader, .exampleblock > .content h3.subheader, .exampleblock > .content .subheader#toctitle, .sidebarblock.exampleblock > .content > .subheader.title, .exampleblock > .content h4.subheader, .exampleblock > .content h5.subheader, .exampleblock > .content h6.subheader {
    line-height: 1.4;
}

.exampleblock.result > .content {
    -webkit-box-shadow: 0 1px 8px var(--toc-border-color);
    box-shadow: 0 1px 8px var(--toc-border-color);
}

.sidebarblock {
    border-style: solid;
    border-width: 1px;
    border-color: var(--toc-border-color);
    margin-bottom: 1.25em;
    padding: 1.25em;
    background: var(--pre-background-color);
    -webkit-border-radius: var(--border-radius-default);
    border-radius: var(--border-radius-default);
}

.sidebarblock > :first-child {
    margin-top: 0;
}

.sidebarblock > :last-child {
    margin-bottom: 0;
}

.sidebarblock h1, .sidebarblock h2, .sidebarblock h3, .sidebarblock #toctitle, .sidebarblock > .content > .title, .sidebarblock h4, .sidebarblock h5, .sidebarblock h6, .sidebarblock p {
    color: var(--dark-gray-color);
}

.sidebarblock h1, .sidebarblock h2, .sidebarblock h3, .sidebarblock #toctitle, .sidebarblock > .content > .title, .sidebarblock h4, .sidebarblock h5, .sidebarblock h6 {
    line-height: 1;
    margin-bottom: 0.625em;
}

.sidebarblock h1.subheader, .sidebarblock h2.subheader, .sidebarblock h3.subheader, .sidebarblock .subheader#toctitle, .sidebarblock > .content > .subheader.title, .sidebarblock h4.subheader, .sidebarblock h5.subheader, .sidebarblock h6.subheader {
    line-height: 1.4;
}

.sidebarblock > .content > .title {
    color: var(--gray-color);
    margin-top: 0;
    line-height: 1.5;
}

.exampleblock > .content > :last-child > :last-child, .exampleblock > .content .olist > ol > li:last-child > :last-child, .exampleblock > .content .ulist > ul > li:last-child > :last-child, .exampleblock > .content .qlist > ol > li:last-child > :last-child, .sidebarblock > .content > :last-child > :last-child, .sidebarblock > .content .olist > ol > li:last-child > :last-child, .sidebarblock > .content .ulist > ul > li:last-child > :last-child, .sidebarblock > .content .qlist > ol > li:last-child > :last-child {
    margin-bottom: 0;
}

.literalblock > .content pre, .listingblock > .content pre {
    background: #eeeeee;
    border-width: 1px;
    border-style: solid;
    border-color: #cccccc;
    -webkit-border-radius: var(--border-radius-default);
    border-radius: var(--border-radius-default);
    padding: 0.5em;
    word-wrap: break-word;
}

.literalblock > .content pre.nowrap, .listingblock > .content pre.nowrap {
    overflow-x: auto;
    white-space: pre;
    word-wrap: normal;
}

.literalblock > .content pre > code, .listingblock > .content pre > code {
    display: block;
}

@media only screen {
    .literalblock > .content pre, .listingblock > .content pre {
        font-size: 0.76em;
    }
}

@media only screen and (min-width: 768px) {
    .literalblock > .content pre, .listingblock > .content pre {
        font-size: 0.855em;
    }
}

@media only screen and (min-width: 1280px) {
    .literalblock > .content pre, .listingblock > .content pre {
        font-size: 0.95em;
    }
}

.listingblock > .content {
    position: relative;
}

.listingblock:hover code[class*=" language-"]:before {
    text-transform: uppercase;
    font-size: 0.9em;
    color: #999;
    position: absolute;
    top: 0.375em;
    right: 0.375em;
}

.listingblock:hover code.asciidoc:before {
    content: "asciidoc";
}

.listingblock:hover code.clojure:before {
    content: "clojure";
}

.listingblock:hover code.css:before {
    content: "css";
}

.listingblock:hover code.groovy:before {
    content: "groovy";
}

.listingblock:hover code.html:before {
    content: "html";
}

.listingblock:hover code.java:before {
    content: "java";
}

.listingblock:hover code.javascript:before {
    content: "javascript";
}

.listingblock:hover code.python:before {
    content: "python";
}

.listingblock:hover code.ruby:before {
    content: "ruby";
}

.listingblock:hover code.scss:before {
    content: "scss";
}

.listingblock:hover code.xml:before {
    content: "xml";
}

.listingblock:hover code.yaml:before {
    content: "yaml";
}

.listingblock.terminal pre .command:before {
    content: attr(data-prompt);
    padding-right: 0.5em;
    color: #999;
}

.listingblock.terminal pre .command:not([data-prompt]):before {
    content: '$';
}

table.pyhltable {
    border: 0;
    margin-bottom: 0;
}

table.pyhltable td {
    vertical-align: top;
    padding-top: 0;
    padding-bottom: 0;
}

table.pyhltable td.code {
    padding-left: .75em;
    padding-right: 0;
}

.highlight.pygments .lineno, table.pyhltable td:not(.code) {
    color: #999;
    padding-left: 0;
    padding-right: .5em;
    border-right: 1px solid var(--border2);
}

.highlight.pygments .lineno {
    display: inline-block;
    margin-right: .25em;
}

table.pyhltable .linenodiv {
    background-color: transparent !important;
    padding-right: 0 !important;
}

.quoteblock {
    margin: 0 0 1.25em;
    padding: 0.5625em 1.25em 0 1.1875em;
    border-left: 1px solid var(--border2);
}

.quoteblock blockquote {
    margin: 0 0 1.25em 0;
    padding: 0 0 0.5625em 0;
    border: 0;
}

.quoteblock blockquote > .paragraph:last-child p {
    margin-bottom: 0;
}

.quoteblock .attribution {
    margin-top: -.25em;
    padding-bottom: 0.5625em;
    font-size: 0.8125em;
    color: var(--block-quote-color);
}

.quoteblock .attribution br {
    display: none;
}

.quoteblock .attribution cite {
    display: block;
    margin-bottom: 0.625em;
}

table thead th, table tfoot th {
    font-weight: bold;
}

table.tableblock.grid-all {
    border-collapse: separate;
    border-spacing: 1px;
    -webkit-border-radius: var(--border-radius-default);
    border-radius: var(--border-radius-default);
    border-top: 0 solid var(--border2);
    border-bottom: 0 solid var(--border2);
}

table.tableblock.frame-topbot, table.tableblock.frame-none {
    border-left: 0;
    border-right: 0;
}

table.tableblock.frame-sides, table.tableblock.frame-none {
    border-top: 0;
    border-bottom: 0;
}

table.tableblock td .paragraph:last-child p, table.tableblock td > p:last-child {
    margin-bottom: 0;
}

th.tableblock.halign-left, td.tableblock.halign-left {
    text-align: left;
}

th.tableblock.halign-right, td.tableblock.halign-right {
    text-align: right;
}

th.tableblock.halign-center, td.tableblock.halign-center {
    text-align: center;
}

th.tableblock.valign-top, td.tableblock.valign-top {
    vertical-align: top;
}

th.tableblock.valign-bottom, td.tableblock.valign-bottom {
    vertical-align: bottom;
}

th.tableblock.valign-middle, td.tableblock.valign-middle {
    vertical-align: middle;
}

p.tableblock.header {
    color: var(--foreground-color);
    font-weight: bold;
}

td > div.verse {
    white-space: pre;
}

ol {
    margin-left: 0.25em;
}

ul li ol {
    margin-left: 0;
}

dl dd {
    margin-left: 1.125em;
}

dl dd:last-child, dl dd:last-child > :last-child {
    margin-bottom: 0;
}

ol > li p, ul > li p, ul dd, ol dd, .olist .olist, .ulist .ulist, .ulist .olist, .olist .ulist {
    margin-bottom: 0.625em;
}

ul.unstyled, ol.unnumbered, ul.checklist, ul.none {
    list-style-type: none;
}

ul.unstyled, ol.unnumbered, ul.checklist {
    margin-left: 0.625em;
}

ul.checklist li > p:first-child > i[class^="icon-check"]:first-child, ul.checklist li > p:first-child > input[type="checkbox"]:first-child {
    margin-right: 0.25em;
}

ul.checklist li > p:first-child > input[type="checkbox"]:first-child {
    position: relative;
    top: 1px;
}

ul.inline {
    margin: 0 auto 0.625em auto;
    margin-left: -1.375em;
    margin-right: 0;
    padding: 0;
    list-style: none;
    overflow: hidden;
}

ul.inline > li {
    list-style: none;
    float: left;
    margin-left: 1.375em;
    display: block;
}

ul.inline > li > * {
    display: block;
}

.unstyled dl dt {
    font-weight: normal;
    font-style: normal;
}

ol.arabic {
    list-style-type: decimal;
}

ol.decimal {
    list-style-type: decimal-leading-zero;
}

ol.loweralpha {
    list-style-type: lower-alpha;
}

ol.upperalpha {
    list-style-type: upper-alpha;
}

ol.lowerroman {
    list-style-type: lower-roman;
}

ol.upperroman {
    list-style-type: upper-roman;
}

ol.lowergreek {
    list-style-type: lower-greek;
}

.hdlist > table, .colist > table {
    border: 0;
    background: none;
}

.hdlist > table > tbody > tr, .colist > table > tbody > tr {
    background: none;
}

td.hdlist1 {
    padding-right: .8em;
    font-weight: bold;
}

td.hdlist1, td.hdlist2 {
    vertical-align: top;
}

.literalblock + .colist, .listingblock + .colist {
    margin-top: -0.5em;
}

.colist > table tr > td:first-of-type {
    padding: 0 .8em;
    line-height: 1;
}

.colist > table tr > td:last-of-type {
    padding: 0.25em 0;
}

.qanda > ol > li > p > em:only-child {
    color: var(--em-color);
}

.thumb, .th {
    line-height: 0;
    display: inline-block;
    border: solid 4px white;
    -webkit-box-shadow: 0 0 0 1px var(--border2);
    box-shadow: 0 0 0 1px var(--border2);
}

.imageblock.left, .imageblock[style*="float: left"] {
    margin: 0.25em 0.625em 1.25em 0;
}

.imageblock.right, .imageblock[style*="float: right"] {
    margin: 0.25em 0 1.25em 0.625em;
}

.imageblock > .title {
    margin-bottom: 0;
}

.imageblock.thumb, .imageblock.th {
    border-width: var(--border-radius-default);
}

.imageblock.thumb > .title, .imageblock.th > .title {
    padding: 0 0.125em;
}

.image.left, .image.right {
    margin-top: 0.25em;
    margin-bottom: 0.25em;
    display: inline-block;
    line-height: 0;
}

.image.left {
    margin-right: 0.625em;
}

.image.right {
    margin-left: 0.625em;
}

a.image {
    text-decoration: none;
}

span.footnote, span.footnoteref {
    vertical-align: super;
    font-size: 0.875em;
}

span.footnote a, span.footnoteref a {
    text-decoration: none;
}

#footnotes {
    padding-top: 0.75em;
    padding-bottom: 0.75em;
    margin-bottom: 0.625em;
}

#footnotes hr {
    width: 20%;
    min-width: 6.25em;
    margin: -.25em 0 .75em 0;
    border-width: 1px 0 0 0;
}

#footnotes .footnote {
    padding: 0 0.375em;
    line-height: 1.3;
    font-size: 0.875em;
    margin-left: 1.2em;
    text-indent: -1.2em;
    margin-bottom: .2em;
}

#footnotes .footnote a:first-of-type {
    font-weight: bold;
    text-decoration: none;
}

#footnotes .footnote:last-of-type {
    margin-bottom: 0;
}

#content #footnotes {
    margin-top: -0.625em;
    margin-bottom: 0;
    padding: 0.75em 0;
}

.gist .file-data > table {
    border: none;
    background: var(--background);
    width: 100%;
    margin-bottom: 0;
}

.gist .file-data > table td.line-data {
    width: 99%;
}

div.unbreakable {
    page-break-inside: avoid;
}

.big {
    font-size: larger;
}

.small {
    font-size: smaller;
}

.underline {
    text-decoration: underline;
}

.overline {
    text-decoration: overline;
}

.line-through {
    text-decoration: line-through;
}

.aqua {
    color: #00bfbf;
}

.aqua-background {
    background-color: #00fafa;
}

.black {
    color: black;
}

.black-background {
    background-color: black;
}

.blue {
    color: #0000bf;
}

.blue-background {
    background-color: #0000fa;
}

.fuchsia {
    color: #bf00bf;
}

.fuchsia-background {
    background-color: #fa00fa;
}

.gray {
    color: #606060;
}

.gray-background {
    background-color: #7d7d7d;
}

.green {
    color: #006000;
}

.green-background {
    background-color: #007d00;
}

.lime {
    color: #00bf00;
}

.lime-background {
    background-color: #00fa00;
}

.maroon {
    color: #600000;
}

.maroon-background {
    background-color: #7d0000;
}

.navy {
    color: #000060;
}

.navy-background {
    background-color: #00007d;
}

.olive {
    color: #606000;
}

.olive-background {
    background-color: #7d7d00;
}

.purple {
    color: #600060;
}

.purple-background {
    background-color: #7d007d;
}

.red {
    color: #bf0000;
}

.red-background {
    background-color: #fa0000;
}

.silver {
    color: #909090;
}

.silver-background {
    background-color: #bcbcbc;
}

.teal {
    color: #006060;
}

.teal-background {
    background-color: #007d7d;
}

.white {
    color: #bfbfbf;
}

.white-background {
    background-color: #fafafa;
}

.yellow {
    color: #bfbf00;
}

.yellow-background {
    background-color: #fafa00;
}

span.icon > [class^="icon-"], span.icon > [class*=" icon-"] {
    cursor: default;
}


.admonitionblock td.icon [class^="icon-"]:before {
    font-size: 2.5em;
    text-shadow: 1px 1px 2px rgba(0, 0, 0, 0.5);
    cursor: default;
}

.admonitionblock td.icon .icon-note:before {
    content: "\f05a";
    color: var(--accent-primary-color);
}

.admonitionblock td.icon .icon-tip:before {
    content: "\f0eb";
    /*text-shadow: 1px 1px 2px var(--accent-primary-color);*/
    color: var(--accent-primary-color);
}

.fa {
    font-family: "FontAwesome", "Helvetica Neue", sans-serif;
}

.admonitionblock td.icon .icon-warning:before {
    content: "\f071";
    color: #bf6900;
}

.admonitionblock td.icon .icon-caution:before {
    content: "\f06d";
    color: #bf3400;
}

.admonitionblock td.icon .icon-important:before {
    content: "\f06a";
    color: #bf0000;
}

.conum {
    display: inline-block;
    color: white !important;
    background-color: black;
    -webkit-border-radius: 100px;
    border-radius: 100px;
    text-align: center;
    width: 20px;
    height: 20px;
    font-size: 12px;
    font-weight: bold;
    line-height: 20px;
    font-family: Arial, sans-serif;
    font-style: normal;
    position: relative;
    top: -2px;
    letter-spacing: -1px;
}

.conum * {
    color: white !important;
}

.conum + b {
    display: none;
}

.conum:after {
    content: attr(data-value);
}

.conum:not([data-value]):empty {
    display: none;
}

h4 {
    color: var(--gray-color);
}

.literalblock > .content > pre, .listingblock > .content > pre {
    -webkit-border-radius: var(--border-radius-default);
    border-radius: var(--border-radius-default);
    margin-left: 0.5em;
    margin-right: 0.5em;
}

.admonitionblock {
    margin-left: .5em;
    margin-right: .5em;
}

.admonitionblock > table {
    border: 1px solid var(--accent-primary-color);
    border-left-width: 4px;
    background-color: var(--admotion-background-color);
    border-collapse: separate;
    -webkit-border-radius: var(--border-radius-default);
    border-radius: var(--border-radius-default);
}

.admonitionblock > table td.icon {
    padding-top: .5em;
    padding-bottom: .5em;
}

.admonitionblock > table td.content {
    padding: .5em .2em;
    color: black;
    font-size: .9em;
    border-left: none;
}

.sidebarblock {
    background-color: var(--sidebar-background-color);
    border-color: #ccc;
}

.sidebarblock > .content > .title {
    color: var(--link-color-default);
}

table.tableblock.grid-all {
    border-collapse: collapse;
    -webkit-border-radius: 0;
    border-radius: 0;
}

table.tableblock.grid-all th.tableblock, table.tableblock.grid-all td.tableblock {
    border-bottom: 1px solid #aaa;
}

#footer {
    background-color: var(--accent-primary-color);
    padding: 2em;
}

#footer-text {
    color: #eee;
    font-size: 0.8em;
    text-align: center;
}
</style>
<style>
pre.rouge table td { padding: 5px; }
pre.rouge table pre { margin: 0; }
pre.rouge, pre.rouge .w {
  color: #24292f;
  background-color: #f6f8fa;
}
pre.rouge .k, pre.rouge .kd, pre.rouge .kn, pre.rouge .kp, pre.rouge .kr, pre.rouge .kt, pre.rouge .kv {
  color: #cf222e;
}
pre.rouge .gr {
  color: #f6f8fa;
}
pre.rouge .gd {
  color: #82071e;
  background-color: #ffebe9;
}
pre.rouge .nb {
  color: #953800;
}
pre.rouge .nc {
  color: #953800;
}
pre.rouge .no {
  color: #953800;
}
pre.rouge .nn {
  color: #953800;
}
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</head>
<body class="article toc2 toc-left">
<div id="header">
<h1>Data Validation</h1>
<div id="toc" class="toc2">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#_before_we_start">Before we start</a></li>
<li><a href="#_approach_1_validate_a_property_using_dataannotations">Approach 1 : Validate a Property using DataAnnotations</a>
<ul class="sectlevel2">
<li><a href="#_step_1_setup_the_viewmodel">Step 1: Setup the ViewModel</a></li>
<li><a href="#_step_2_add_the_property">Step 2: Add the Property</a></li>
<li><a href="#_step_3_setup_the_view">Step 3: Setup the View</a></li>
<li><a href="#_step_4_run_the_app">Step 4: Run the App</a></li>
</ul>
</li>
<li><a href="#_approach_2_validate_a_property_using_inotifydataerrorinfo">Approach 2 : Validate a Property using INotifyDataErrorInfo</a>
<ul class="sectlevel2">
<li><a href="#_step_1_setup_the_viewmodel_2">Step 1: Setup the ViewModel</a></li>
<li><a href="#_step_2_add_the_property_2">Step 2: Add the Property</a></li>
<li><a href="#_step_3_setup_the_view_2">Step 3: Setup the View</a></li>
<li><a href="#_step_4_run_the_app_2">Step 4: Run the App</a></li>
</ul>
</li>
<li><a href="#_approach_3_validate_a_property_using_exceptions_inside_setters">Approach 3 : Validate a Property using Exceptions inside setters</a>
<ul class="sectlevel2">
<li><a href="#_step_1_setup_the_viewmodel_3">Step 1: Setup the ViewModel</a></li>
<li><a href="#_step_2_add_the_property_3">Step 2: Add the Property</a></li>
<li><a href="#_step_3_setup_the_view_3">Step 3: Setup the View</a></li>
<li><a href="#_step_4_run_the_app_3">Step 4: Run the App</a></li>
</ul>
</li>
<li><a href="#_related">Related</a></li>
</ul>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p>This example will show you how to <a href="https://docs.avaloniaui.net/docs/guides/development-guides/data-validation">[validate properties]</a> and display an error message to the user, if the entered values are invalid. For example if the user should enter their e-mail address into a <code>TextBox</code>, the "@"-character is required and the <code>TextBox</code> may not be empty.</p>
</div>
<!-- toc disabled -->
<h3 id="_difficulty" class="discrete">Difficulty</h3>
<div class="paragraph">
<p>🐥 Easy 🐥</p>
</div>
<h3 id="_buzz_words" class="discrete">Buzz-Words</h3>
<div class="paragraph">
<p>Data-Annotation, Data-Validation, Exception, Error, Error-Message, Binding, MVVM</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_before_we_start">Before we start</h2>
<div class="sectionbody">
<div class="paragraph">
<p>You should already know how to setup a new Avalonia project. Moreover you should know the basics about the MVVM-pattern and how to bind a property of your <code>ViewModel</code> to your <code>View</code>. In this sample we will show you three options to validate a property. It&#8217;s up to you, which one you prefer. You can also combine them.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_approach_1_validate_a_property_using_dataannotations">Approach 1 : Validate a Property using DataAnnotations</h2>
<div class="sectionbody">
<div class="paragraph">
<p>In the first approach we will show you is how to use <a href="https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.dataannotations.validationattribute">[<code>DataAnnotations</code>]</a> to validate a property. To get started, please create a new project using the <code>Avalonia MVVM Template</code>.</p>
</div>
<div class="sect2">
<h3 id="_step_1_setup_the_viewmodel">Step 1: Setup the ViewModel</h3>
<div class="paragraph">
<p>As we want to show different validation options, we add another file called <code>ValidationUsingDataAnnotationViewModel</code> into the <code>ViewModels</code>-folder. This class will be added as a read-only Property into our <code>MainWindowViewModel</code>:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="cs"><span class="c1">/// &lt;summary&gt;</span>
<span class="c1">/// Gets a ViewModel showing how to use DataAnnotations for validation</span>
<span class="c1">/// &lt;/summary&gt;</span>
<span class="k">public</span> <span class="n">ValidationUsingDataAnnotationViewModel</span> <span class="n">ValidationUsingDataAnnotationViewModel</span> <span class="p">{</span> <span class="k">get</span><span class="p">;</span> <span class="p">}</span>
    <span class="p">=</span> <span class="k">new</span> <span class="nf">ValidationUsingDataAnnotationViewModel</span><span class="p">();</span></code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_step_2_add_the_property">Step 2: Add the Property</h3>
<div class="paragraph">
<p>Let&#8217;s assume we want to make a registration-form where an e-mail-address is required and must be a valid e-mail-address. We add a property called <code>EMail</code>. This property will be decorated with the <a href="https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.dataannotations.requiredattribute">[<code>RequiredAttribute</code>]</a> and the <a href="https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.dataannotations.emailaddressattribute">[<code>EMailAddressAttribute</code>]</a>. You can combine as many attributes as you want as well as creating your own <a href="https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.dataannotations.validationattribute">[<code>ValidationAttribute</code>]</a>.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="cs"><span class="k">private</span> <span class="kt">string</span><span class="p">?</span> <span class="n">_EMail</span><span class="p">;</span>

<span class="c1">/// &lt;summary&gt;</span>
<span class="c1">/// Validation using DataAnnotation</span>
<span class="c1">/// &lt;/summary&gt;</span>
<span class="p">[</span><span class="n">Required</span><span class="p">]</span>
<span class="p">[</span><span class="n">EmailAddress</span><span class="p">]</span>
<span class="k">public</span> <span class="kt">string</span><span class="p">?</span> <span class="n">EMail</span>
<span class="p">{</span>
    <span class="k">get</span> <span class="p">{</span> <span class="k">return</span> <span class="n">_EMail</span><span class="p">;</span> <span class="p">}</span>
    <span class="k">set</span> <span class="p">{</span> <span class="k">this</span><span class="p">.</span><span class="nf">RaiseAndSetIfChanged</span><span class="p">(</span><span class="k">ref</span> <span class="n">_EMail</span><span class="p">,</span> <span class="k">value</span><span class="p">);</span> <span class="p">}</span>
<span class="p">}</span></code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_step_3_setup_the_view">Step 3: Setup the View</h3>
<div class="paragraph">
<p>In the file <code>MainWindow.axaml</code> we add a <code>TabControl</code> where each <code>TabItem</code> represents one of our options. Inside the <code>TabItem</code> there is just one <code>TextBox</code> which binds to our property <code>EMail</code>.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="xml"><span class="nt">&lt;TabControl</span> <span class="na">TabStripPlacement=</span><span class="s">"Bottom"</span><span class="nt">&gt;</span>
    <span class="c">&lt;!--Validation using DataAnnotations--&gt;</span>
    <span class="nt">&lt;TabItem</span> <span class="na">Header=</span><span class="s">"DataAnnotations"</span><span class="nt">&gt;</span>
        <span class="nt">&lt;TextBox</span> <span class="na">Text=</span><span class="s">"{Binding ValidationUsingDataAnnotationViewModel.EMail}"</span> <span class="nt">/&gt;</span>
    <span class="nt">&lt;/TabItem&gt;</span>
<span class="nt">&lt;/TabControl&gt;</span></code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_step_4_run_the_app">Step 4: Run the App</h3>
<div class="paragraph">
<p>In your IDE hit <code>Run</code> or <code>Debug</code> to see the result:</p>
</div>
<div class="imageblock">
<div class="content">
<img src="_docs/option1.png" alt="Result Option 1">
</div>
<div class="title">Figure 1. Result Option 1</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_approach_2_validate_a_property_using_inotifydataerrorinfo">Approach 2 : Validate a Property using INotifyDataErrorInfo</h2>
<div class="sectionbody">
<div class="paragraph">
<p>You can also implement the interface <a href="https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.inotifydataerrorinfo">[<code>INotifyDataErrorInfo</code>]</a> into your <code>ViewModel</code> or it&#8217;s base class. Some MVVM-libraries already implement this interface by default.</p>
</div>
<div class="sect2">
<h3 id="_step_1_setup_the_viewmodel_2">Step 1: Setup the ViewModel</h3>
<div class="paragraph">
<p>Again we add another file called <code>ValidationUsingINotifyDataErrorInfoViewModel</code> into the <code>ViewModels</code>-folder. This class will be added as a read-only Property into our <code>MainWindowViewModel</code>:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="cs"><span class="c1">/// &lt;summary&gt;</span>
<span class="c1">/// Gets a ViewModel showing how to use INotifyDataErrorInfo for validation</span>
<span class="c1">/// &lt;/summary&gt;</span>
<span class="k">public</span> <span class="n">ValidationUsingINotifyDataErrorInfoViewModel</span> <span class="n">ValidationUsingINotifyDataErrorInfoViewModel</span> <span class="p">{</span> <span class="k">get</span><span class="p">;</span> <span class="p">}</span>
    <span class="p">=</span> <span class="k">new</span> <span class="nf">ValidationUsingINotifyDataErrorInfoViewModel</span><span class="p">();</span></code></pre>
</div>
</div>
<div class="paragraph">
<p>Now we have to implement the interface, if this is not already done by the base-class you use. In our sample we add the interface on our own. The interface requires these members:</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<colgroup>
<col style="width: 100%;">
</colgroup>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><div class="content"><div class="dlist">
<dl>
<dt class="hdlist1">bool HasErrors</dt>
<dd>
<p>Gets if there are any errors.</p>
</dd>
<dt class="hdlist1">void GetErrors(string? propertyName)</dt>
<dd>
<p>Gets the errors for a given property (by its name) or all errors, if property name was <code>null</code> or <code>string.Empty</code>.</p>
</dd>
<dt class="hdlist1">event ErrorsChanged</dt>
<dd>
<p>this event is raised whenever any error was added or removed.</p>
</dd>
</dl>
</div></div></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>In addition to the interface members we add the following items:</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<colgroup>
<col style="width: 100%;">
</colgroup>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><div class="content"><div class="dlist">
<dl>
<dt class="hdlist1">private Dictionary errors</dt>
<dd>
<p>A <code>Dictionary</code> that caches the errors, so we don&#8217;t need to validate the properties every time.</p>
</dd>
<dt class="hdlist1">protected void ClearErrors(string? propertyName = null)</dt>
<dd>
<p>This void will clear all errors for the given property name or all errors if <code>propertyName</code> was <code>null</code>. The <code>ErrorsChanged</code>-event will be raised.</p>
</dd>
<dt class="hdlist1">protected void AddError(string propertyName, string errorMessage)</dt>
<dd>
<p>This void will add a given error message for a given property by it&#8217;s name. The <code>ErrorsChanged</code>-event will be raised.</p>
</dd>
</dl>
</div></div></td>
</tr>
</tbody>
</table>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
the <code>protected</code> keyword helps us to make the properties or methods available in derived classes, but not from any external class.
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>Below you can see the final implementation:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="cs"><span class="k">public</span> <span class="k">class</span> <span class="nc">ValidationUsingINotifyDataErrorInfoViewModel</span> <span class="p">:</span> <span class="n">ViewModelBase</span><span class="p">,</span> <span class="n">INotifyDataErrorInfo</span>
<span class="p">{</span>
    <span class="c1">// Implement members of INotifyDataErrorInfo</span>

    <span class="k">public</span> <span class="k">event</span> <span class="n">EventHandler</span><span class="p">&lt;</span><span class="n">DataErrorsChangedEventArgs</span><span class="p">&gt;?</span> <span class="n">ErrorsChanged</span><span class="p">;</span>

    <span class="c1">// we have errors present if errors.Count is greater than 0</span>
    <span class="k">public</span> <span class="kt">bool</span> <span class="n">HasErrors</span> <span class="p">=&gt;</span> <span class="n">errors</span><span class="p">.</span><span class="n">Count</span> <span class="p">&gt;</span> <span class="m">0</span><span class="p">;</span>

    <span class="c1">/// &lt;inheritdoc /&gt;</span>
    <span class="k">public</span> <span class="n">IEnumerable</span> <span class="nf">GetErrors</span><span class="p">(</span><span class="kt">string</span><span class="p">?</span> <span class="n">propertyName</span><span class="p">)</span>
    <span class="p">{</span>
        <span class="c1">// Get entity-level errors when the target property is null or empty</span>
        <span class="k">if</span> <span class="p">(</span><span class="kt">string</span><span class="p">.</span><span class="nf">IsNullOrEmpty</span><span class="p">(</span><span class="n">propertyName</span><span class="p">))</span>
        <span class="p">{</span>
            <span class="k">return</span> <span class="n">errors</span><span class="p">.</span><span class="n">Values</span><span class="p">.</span><span class="nf">SelectMany</span><span class="p">(</span><span class="k">static</span> <span class="n">errors</span> <span class="p">=&gt;</span> <span class="n">errors</span><span class="p">);</span>
        <span class="p">}</span>

        <span class="c1">// Property-level errors, if any</span>
        <span class="k">if</span> <span class="p">(</span><span class="k">this</span><span class="p">.</span><span class="n">errors</span><span class="p">.</span><span class="nf">TryGetValue</span><span class="p">(</span><span class="n">propertyName</span><span class="p">!,</span> <span class="k">out</span> <span class="n">List</span><span class="p">&lt;</span><span class="n">ValidationResult</span><span class="p">&gt;?</span> <span class="n">result</span><span class="p">))</span>
        <span class="p">{</span>
            <span class="k">return</span> <span class="n">result</span><span class="p">;</span>
        <span class="p">}</span>

        <span class="c1">// In case there are no errors we return an empty array.</span>
        <span class="k">return</span> <span class="n">Array</span><span class="p">.</span><span class="n">Empty</span><span class="p">&lt;</span><span class="n">ValidationResult</span><span class="p">&gt;();</span>
    <span class="p">}</span>

    <span class="c1">// Store Errors in a Dictionary</span>
    <span class="k">private</span> <span class="n">Dictionary</span><span class="p">&lt;</span><span class="kt">string</span><span class="p">,</span> <span class="n">List</span><span class="p">&lt;</span><span class="n">ValidationResult</span><span class="p">&gt;&gt;</span> <span class="n">errors</span> <span class="p">=</span> <span class="k">new</span> <span class="n">Dictionary</span><span class="p">&lt;</span><span class="kt">string</span><span class="p">,</span> <span class="n">List</span><span class="p">&lt;</span><span class="n">ValidationResult</span><span class="p">&gt;&gt;();</span>

    <span class="c1">/// &lt;summary&gt;</span>
    <span class="c1">/// Clears the errors for a given property name.</span>
    <span class="c1">/// &lt;/summary&gt;</span>
    <span class="c1">/// &lt;param name="propertyName"&gt;The name of the property to clear or all properties if &lt;see langword="null"/&gt;&lt;/param&gt;</span>
    <span class="k">protected</span> <span class="k">void</span> <span class="nf">ClearErrors</span><span class="p">(</span><span class="kt">string</span><span class="p">?</span> <span class="n">propertyName</span> <span class="p">=</span> <span class="k">null</span><span class="p">)</span>
    <span class="p">{</span>
        <span class="c1">// Clear entity-level errors when the target property is null or empty</span>
        <span class="k">if</span> <span class="p">(</span><span class="kt">string</span><span class="p">.</span><span class="nf">IsNullOrEmpty</span><span class="p">(</span><span class="n">propertyName</span><span class="p">))</span>
        <span class="p">{</span>
            <span class="n">errors</span><span class="p">.</span><span class="nf">Clear</span><span class="p">();</span>
        <span class="p">}</span>
        <span class="k">else</span>
        <span class="p">{</span>
            <span class="n">errors</span><span class="p">.</span><span class="nf">Remove</span><span class="p">(</span><span class="n">propertyName</span><span class="p">);</span>
        <span class="p">}</span>

        <span class="c1">// Notify that errors have changed</span>
        <span class="n">ErrorsChanged</span><span class="p">?.</span><span class="nf">Invoke</span><span class="p">(</span><span class="k">this</span><span class="p">,</span> <span class="k">new</span> <span class="nf">DataErrorsChangedEventArgs</span><span class="p">(</span><span class="n">propertyName</span><span class="p">));</span>
        <span class="k">this</span><span class="p">.</span><span class="nf">RaisePropertyChanged</span><span class="p">(</span><span class="k">nameof</span><span class="p">(</span><span class="n">HasErrors</span><span class="p">));</span>
    <span class="p">}</span>

    <span class="c1">/// &lt;summary&gt;</span>
    <span class="c1">/// Adds a given error message for a given property name.</span>
    <span class="c1">/// &lt;/summary&gt;</span>
    <span class="c1">/// &lt;param name="propertyName"&gt;the name of the property&lt;/param&gt;</span>
    <span class="c1">/// &lt;param name="errorMessage"&gt;The error message to show&lt;/param&gt;</span>
    <span class="k">protected</span> <span class="k">void</span> <span class="nf">AddError</span><span class="p">(</span><span class="kt">string</span> <span class="n">propertyName</span><span class="p">,</span> <span class="kt">string</span> <span class="n">errorMessage</span><span class="p">)</span>
    <span class="p">{</span>
        <span class="c1">// Add the cached errors list for later use.</span>
        <span class="k">if</span> <span class="p">(!</span><span class="n">errors</span><span class="p">.</span><span class="nf">TryGetValue</span><span class="p">(</span><span class="n">propertyName</span><span class="p">,</span> <span class="k">out</span> <span class="n">List</span><span class="p">&lt;</span><span class="n">ValidationResult</span><span class="p">&gt;?</span> <span class="n">propertyErrors</span><span class="p">))</span>
        <span class="p">{</span>
            <span class="n">propertyErrors</span> <span class="p">=</span> <span class="k">new</span> <span class="n">List</span><span class="p">&lt;</span><span class="n">ValidationResult</span><span class="p">&gt;();</span>
            <span class="n">errors</span><span class="p">.</span><span class="nf">Add</span><span class="p">(</span><span class="n">propertyName</span><span class="p">,</span> <span class="n">propertyErrors</span><span class="p">);</span>
        <span class="p">}</span>

        <span class="n">propertyErrors</span><span class="p">.</span><span class="nf">Add</span><span class="p">(</span><span class="k">new</span> <span class="nf">ValidationResult</span><span class="p">(</span><span class="n">errorMessage</span><span class="p">));</span>

        <span class="c1">// Notify that errors have changed</span>
        <span class="n">ErrorsChanged</span><span class="p">?.</span><span class="nf">Invoke</span><span class="p">(</span><span class="k">this</span><span class="p">,</span> <span class="k">new</span> <span class="nf">DataErrorsChangedEventArgs</span><span class="p">(</span><span class="n">propertyName</span><span class="p">));</span>
        <span class="k">this</span><span class="p">.</span><span class="nf">RaisePropertyChanged</span><span class="p">(</span><span class="k">nameof</span><span class="p">(</span><span class="n">HasErrors</span><span class="p">));</span>
    <span class="p">}</span>
<span class="p">}</span></code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_step_2_add_the_property_2">Step 2: Add the Property</h3>
<div class="paragraph">
<p>The property is implemented like any other property of our <code>ViewModel</code>:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="cs"><span class="k">private</span> <span class="kt">string</span><span class="p">?</span> <span class="n">_EMail</span><span class="p">;</span>

<span class="c1">/// &lt;summary&gt;</span>
<span class="c1">/// A property that is validated using INotifyDataErrorInfo</span>
<span class="c1">/// &lt;/summary&gt;</span>
<span class="k">public</span> <span class="kt">string</span><span class="p">?</span> <span class="n">EMail</span>
<span class="p">{</span>
    <span class="k">get</span> <span class="p">{</span> <span class="k">return</span> <span class="n">_EMail</span><span class="p">;</span> <span class="p">}</span>
    <span class="k">set</span> <span class="p">{</span> <span class="k">this</span><span class="p">.</span><span class="nf">RaiseAndSetIfChanged</span><span class="p">(</span><span class="k">ref</span> <span class="n">_EMail</span><span class="p">,</span> <span class="k">value</span><span class="p">);</span> <span class="p">}</span>
<span class="p">}</span></code></pre>
</div>
</div>
<div class="paragraph">
<p>In addition we will add a method called <code>Validate_EMail</code> that will validate our property:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="cs"><span class="k">private</span> <span class="k">void</span> <span class="nf">Validate_EMail</span><span class="p">()</span>
<span class="p">{</span>
    <span class="c1">// first of all clear all previous errors</span>
    <span class="nf">ClearErrors</span><span class="p">(</span><span class="k">nameof</span><span class="p">(</span><span class="n">EMail</span><span class="p">));</span>

    <span class="c1">// No empty string allowed</span>
    <span class="k">if</span> <span class="p">(</span><span class="kt">string</span><span class="p">.</span><span class="nf">IsNullOrEmpty</span><span class="p">(</span><span class="n">EMail</span><span class="p">))</span>
    <span class="p">{</span>
        <span class="nf">AddError</span><span class="p">(</span><span class="k">nameof</span><span class="p">(</span><span class="n">EMail</span><span class="p">),</span> <span class="s">"This field is required"</span><span class="p">);</span>
    <span class="p">}</span>

    <span class="c1">// @-sign required</span>
    <span class="k">if</span> <span class="p">(</span><span class="n">EMail</span> <span class="k">is</span> <span class="k">null</span> <span class="p">||</span> <span class="p">!</span><span class="n">EMail</span><span class="p">.</span><span class="nf">Contains</span><span class="p">(</span><span class="sc">'@'</span><span class="p">))</span>
    <span class="p">{</span>
        <span class="nf">AddError</span><span class="p">(</span><span class="k">nameof</span><span class="p">(</span><span class="n">EMail</span><span class="p">),</span> <span class="s">"Don't forget the '@'-sign"</span><span class="p">);</span>
    <span class="p">}</span>
<span class="p">}</span></code></pre>
</div>
</div>
<div class="paragraph">
<p>In the constructor of our <code>ViewModel</code> we will listen to changes of the property <code>EMail</code> and run the validation with every change. Moreover we will run the validation inside the constructor in order to make sure the initial value is already validated.</p>
</div>
<div class="admonitionblock tip">
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<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
If you want to avoid using <code>WhenAnyValue</code> you can also call the validation inside the properties setter.
</td>
</tr>
</table>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="cs"><span class="k">public</span> <span class="nf">ValidationUsingINotifyDataErrorInfoViewModel</span><span class="p">()</span>
<span class="p">{</span>
    <span class="c1">// Listen to changes of "ValidationUsingINotifyDataErrorInfo" and re-evaluate the validation</span>
    <span class="k">this</span><span class="p">.</span><span class="nf">WhenAnyValue</span><span class="p">(</span><span class="n">x</span> <span class="p">=&gt;</span> <span class="n">x</span><span class="p">.</span><span class="n">EMail</span><span class="p">)</span>
        <span class="p">.</span><span class="nf">Subscribe</span><span class="p">(</span><span class="n">_</span> <span class="p">=&gt;</span> <span class="nf">Validate_EMail</span><span class="p">());</span>

    <span class="c1">// run INotifyDataErrorInfo-validation on start-up</span>
    <span class="nf">Validate_EMail</span><span class="p">();</span>
<span class="p">}</span></code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_step_3_setup_the_view_2">Step 3: Setup the View</h3>
<div class="paragraph">
<p>Let&#8217;s add another <code>TabItem</code> into our <code>TabControl</code>:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="xml"><span class="nt">&lt;TabControl</span> <span class="na">TabStripPlacement=</span><span class="s">"Bottom"</span><span class="nt">&gt;</span>
    <span class="c">&lt;!--Validation using INotifyDataErrorInfo--&gt;</span>
    <span class="nt">&lt;TabItem</span> <span class="na">Header=</span><span class="s">"INotifyDataErrorInfo"</span><span class="nt">&gt;</span>
        <span class="nt">&lt;TextBox</span> <span class="na">Text=</span><span class="s">"{Binding ValidationUsingINotifyDataErrorInfoViewModel.EMail}"</span> <span class="nt">/&gt;</span>
    <span class="nt">&lt;/TabItem&gt;</span>
<span class="nt">&lt;/TabControl&gt;</span></code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_step_4_run_the_app_2">Step 4: Run the App</h3>
<div class="paragraph">
<p>In your IDE hit <code>Run</code> or <code>Debug</code> to see the result:</p>
</div>
<div class="imageblock">
<div class="content">
<img src="_docs/option2.png" alt="Result Option 2">
</div>
<div class="title">Figure 2. Result Option 2</div>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
Some <code>MVVM</code>-frameworks like the <a href="https://learn.microsoft.com/en-us/windows/communitytoolkit/mvvm/observablevalidator">[Windows MVVM Community Toolkit]</a> implement the <code>INotifyDataErrorInfo</code>-interface and are using <a href="https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.dataannotations.validationattribute">[<code>DataAnnotations</code>]</a> to run the validation. In this case you may end up seeing your validation twice. If you are facing such an issue, you can <a href="https://docs.avaloniaui.net/docs/guides/development-guides/data-validation#manage-validationplugins">[disable one of the validators]</a> in your App.
</td>
</tr>
</table>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_approach_3_validate_a_property_using_exceptions_inside_setters">Approach 3 : Validate a Property using Exceptions inside setters</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The third option we want to show you is throwing an <a href="https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/exceptions/">[Exception]</a> inside the setter of your property.</p>
</div>
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<i class="fa icon-important" title="Important"></i>
</td>
<td class="content">
This only works inside setters. Exceptions inside the getter of your property are not allowed and will result in a crash of your application.
</td>
</tr>
</table>
</div>
<div class="sect2">
<h3 id="_step_1_setup_the_viewmodel_3">Step 1: Setup the ViewModel</h3>
<div class="paragraph">
<p>Once again we add another file into the <code>ViewModels</code>-folder, called <code>ValidationUsingExceptionInsideSetterViewModel</code>. As before, also this class is added as a read-only property into our <code>MainWindowViewModel</code>:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="cs"><span class="c1">/// &lt;summary&gt;</span>
<span class="c1">/// Gets a ViewModel showing how to use Exceptions inside the setter for validation</span>
<span class="c1">/// &lt;/summary&gt;</span>
<span class="k">public</span> <span class="n">ValidationUsingExceptionInsideSetterViewModel</span> <span class="n">ValidationUsingExceptionInsideSetterViewModel</span> <span class="p">{</span> <span class="k">get</span><span class="p">;</span> <span class="p">}</span>
    <span class="p">=</span> <span class="k">new</span> <span class="nf">ValidationUsingExceptionInsideSetterViewModel</span><span class="p">();</span></code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_step_2_add_the_property_3">Step 2: Add the Property</h3>
<div class="paragraph">
<p>As written above we can throw exceptions inside the setter of a property. We will have a set of <code>if</code>-conditions, where we throw an <code>Exception</code> under a certain condition.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="cs"><span class="k">public</span> <span class="k">class</span> <span class="nc">ValidationUsingExceptionInsideSetterViewModel</span> <span class="p">:</span> <span class="n">ViewModelBase</span>
<span class="p">{</span>

    <span class="k">private</span> <span class="kt">string</span><span class="p">?</span> <span class="n">_EMail</span><span class="p">;</span>

    <span class="c1">/// &lt;summary&gt;</span>
    <span class="c1">/// Validation using Exceptions (only inside setter allowed!)</span>
    <span class="c1">/// &lt;/summary&gt;</span>
    <span class="k">public</span> <span class="kt">string</span><span class="p">?</span> <span class="n">EMail</span>
    <span class="p">{</span>
        <span class="k">get</span> <span class="p">{</span> <span class="k">return</span> <span class="n">_EMail</span><span class="p">;</span> <span class="p">}</span>
        <span class="k">set</span>
        <span class="p">{</span>
            <span class="c1">// The field may not be null or empty</span>
            <span class="k">if</span> <span class="p">(</span><span class="kt">string</span><span class="p">.</span><span class="nf">IsNullOrEmpty</span><span class="p">(</span><span class="k">value</span><span class="p">))</span>
            <span class="p">{</span>
                <span class="k">throw</span> <span class="k">new</span> <span class="nf">ArgumentNullException</span><span class="p">(</span><span class="k">nameof</span><span class="p">(</span><span class="n">EMail</span><span class="p">),</span> <span class="s">"This field is required"</span><span class="p">);</span>
            <span class="p">}</span>
            <span class="c1">// The field must contain an '@' sign</span>
            <span class="k">else</span> <span class="k">if</span> <span class="p">(!</span><span class="k">value</span><span class="p">.</span><span class="nf">Contains</span><span class="p">(</span><span class="sc">'@'</span><span class="p">))</span>
            <span class="p">{</span>
                <span class="k">throw</span> <span class="k">new</span> <span class="nf">ArgumentException</span><span class="p">(</span><span class="k">nameof</span><span class="p">(</span><span class="n">EMail</span><span class="p">),</span> <span class="s">"Not a valid E-Mail-Address"</span><span class="p">);</span>
            <span class="p">}</span>
            <span class="c1">// The checks were successful, so we can store the value</span>
            <span class="k">else</span>
            <span class="p">{</span>
                <span class="k">this</span><span class="p">.</span><span class="nf">RaiseAndSetIfChanged</span><span class="p">(</span><span class="k">ref</span> <span class="n">_EMail</span><span class="p">,</span> <span class="k">value</span><span class="p">);</span>
            <span class="p">}</span>
        <span class="p">}</span>
    <span class="p">}</span>
<span class="p">}</span></code></pre>
</div>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
<div class="paragraph">
<p>Depending on your IDE-Settings the debugger may stop execution when the <code>Exception</code> is hit. You can safely click <code>Resume</code> or <code>Mute and Resume</code> since the exception will be handled by Avalonia.</p>
</div>
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="_step_3_setup_the_view_3">Step 3: Setup the View</h3>
<div class="paragraph">
<p>We add another <code>TabItem</code> into our <code>TabControl</code>.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="xml"><span class="nt">&lt;TabControl</span> <span class="na">TabStripPlacement=</span><span class="s">"Bottom"</span><span class="nt">&gt;</span>
    <span class="c">&lt;!--Validation using Exceptions in setter--&gt;</span>
    <span class="nt">&lt;TabItem</span> <span class="na">Header=</span><span class="s">"ExceptionInsideSetter"</span><span class="nt">&gt;</span>
        <span class="nt">&lt;TextBox</span> <span class="na">Text=</span><span class="s">"{Binding ValidationUsingExceptionInsideSetterViewModel.EMail}"</span> <span class="nt">/&gt;</span>
    <span class="nt">&lt;/TabItem&gt;</span>
<span class="nt">&lt;/TabControl&gt;</span></code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_step_4_run_the_app_3">Step 4: Run the App</h3>
<div class="paragraph">
<p>In your IDE hit <code>Run</code> or <code>Debug</code> to see the result:</p>
</div>
<div class="imageblock">
<div class="content">
<img src="_docs/option3.png" alt="Result Option 3">
</div>
<div class="title">Figure 3. Result Option 3</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_related">Related</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Below please find a collection with helpful links:</p>
</div>
<div class="ulist">
<ul>
<li>
<p><a href="https://docs.avaloniaui.net/docs/data-binding/data-validation#customize-the-appearance-of-the-validation-message">How to configure the appearance of the error message</a></p>
</li>
<li>
<p><a href="https://docs.avaloniaui.net/docs/data-binding/data-validation#manage-validationplugins">How to add or remove a certain validation option</a></p>
</li>
<li>
<p><a href="https://docs.avaloniaui.net/docs/authoring-controls/defining-properties#datavalidation-support">Define a custom property with validation support in a custom control</a></p>
</li>
</ul>
</div>
</div>
</div>
</div>
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Last updated 2025-11-06 19:42:36 UTC
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